The invention relates to an apparatus for the high-frequency treatment of body tissue with a head which comprises at least one HF electrode of an electrically conductive material, which can be brought into contact with the tissue to be treated.
Such an apparatus for the high-frequency treatment, in particular coagulation, is known from DE 3930451 A1. The head of the apparatus shown therein has a front electrode at the distal end and a rear electrode spaced therefrom in the proximal direction. An insulating ring of a synthetic material is arranged between the electrodes.
DE 3511107 C2 also describes an apparatus for the high-frequency coagulation of biological tissue. There, the legs of a pair of coagulation forceps is formed from an electrically conductive material.
In the apparatus for the high-frequency treatment of body tissue (HF coagulation apparatus) described in DE 3838840 C2, the head is formed by a jacket of poorly heat conducting steel, upon which an Al/Ag layer is applied as an electrode.
DE 3510586 C2 also describes a high-frequency surgical device. Therein a monopolar active electrode is provided which cooperates with a neutral electrode. Alternatively, a bipolar electrode is also provided.
With respect to the state of the art, reference is also made to the following documents: EP 0 246 350 A1; GB 2308979 A; DE 30 50 386 C2; and WO 95/10978 A1.
The present invention can be utilized both with monopolar electrodes and with bipolar electrodes.
In the following, the above mentioned state of the art is taken as known, in particular the electrical supply and the control of the the HF energy.
The present invention relates to a special configuration of the head of an HF surgical device. The term xe2x80x98headxe2x80x99 refers to that component by means of which the HF energy is coupled (transferred) into the body tissue to be treated. The head carries the HF electrodes. So-called cardiac catheters are special apparatuses for the high-frequency treatment of body tissue. The present invention relates in particular to such cardiac catheters as well.
In recent years, the development of the state of the art was directed in particular at increasing the volume of the tissue denaturated by means of HF energy. This applies in particular to the development of the so-called cardiac catheters. For this purpose, the active electrodes of the head have been enlarged. With an increasing electrode diameter and increasing electrode areas, the volume of the denaturated or coagulated, respectively, tissue could be increased. An increase of the HF energy emitted by the HF electrodes, however, is only possible to such a degree that the boundary layer between the HF electrode and the treated tissue does not exceed certain temperatures. The temperature generated in the contact zone between the electrode and the tissue in turn depends on how the generating heat is dissipated from this zone. The electrode itself plays an essential role in this heat dissipation. The tissue through which blood circulates and the blood flowing around the electrode, also dissipate heat. By enlarging the active electrode of the HF head, the contact area with the tissue and thus the cooling area can be enlarged. An increase of the electrode area, however, is not desired for a plurality of applications.
Another possibility of cooling would be the use of so-called active cooling systems such as, for example, flushing-of the electrode with a cooling liquid such as a saline solution. The cooling liquid could be introduced into the patient""s blood stream through small openings in the electrode. This, however, will strain the blood circulation of the patient and increase the risk of infection. In addition, this will make handling of the HF device considerably more cumbersome.
Upon an increase of the electrode areas the blood or the tissue parts, respectively, will be largely heated in an uncontrolled manner by the emission of the HF energy, i.e. blood and tissue parts are heated considerably, for which this is not desired. The blood flowing around the electrode areas can start boiling. Via imaging methods (e.g. ultrasonics) an extreme formation of bubbles at portions of the electrode which were not in direct contact with the tissue to be treated was observed. Such a bubble formation in the blood or at tissue surfaces as well can lead to massive artefacts in the. ECG system during the cardiac ablation. A destruction of blood cells and a formation of coagel are also to be feared. In brain operations and operations on the heart (ablation technique) the phenomena described lead to a considerable interference of signals.
The invention is based on the object to further develop an apparatus for the high-frequency treatment of body tissue of the initially mentioned type in such a manner that a destruction of blood-cells and an undesired formation of coagel as well as blood clotting are avoided. In addition, the efficiency of the apparatus, i.e. the ratio of emitted HF energy to the desired denaturated tissue volume, is to be improved.
For the solution of these technical problems the invention provides for the head supporting the HF electrode to comprise highly heat conductive zones in order to dissipate heat generating at the HF electrode away from the electrode in the proximal direction, with the head having a length of 6 to 12 mm and a diameter of 1.5 to 4 mm.
Thereby the occurrence of temperature peaks near the electrode is prevented and the heat is dissipated away from the critical places. The heat dissipation can also be effected partially in the radial direction (relative to the longitudinal axis of the head).
The invention enables the use of relatively small active electrodes. This enables a precise derivation of small electrical nerve signals and a more precise locating of nerve paths or vessels, respectively. In addition, a high precision with respect to the site of the tissue impedance change during the HF emission is achieved. Furthermore, the invention enables an improvement of the derivation signal quality before and during the HF emission. Temperature measurement is also considerably improved, and the speed of temperature measurement in the active electrode is increased. Finally, the invention also enables an adaptation of the electrode areas to the tissue portions to be treated.
According to the invention, the active electrode of the head of the HF device is preferably no longer formed by a single solid metal body as in the state of the art, which homogeneously emits HF energy in all directions, but by one or several smaller conductive electrode areas which confine themselves to the respective desired emission zone. Thereby, a precisely controllable emission of HF energy into the tissue portion which is actually to be treated is possible. Blood and tissue portions which are not to be subjected to HF energy will be heated to a lesser extent. Cooling is improved by enlarging the overall surface of the head by an electrically insulating but a thermally highly conductive material. Thereby the active electrode area is cooled as well.
For example, the inventive head with the HF electrode or several HF electrodes can be manufactured in such a manner that the head is formed by a solid basic body of a thermally highly conductive material such as diamond or ceramic, onto which the active electrode areas are applied as layers, e.g. by vapour deposition. The active electrode areas can also be formed directly as temperature sensors (e.g. as thermocouples).
Alternatively, the inventive head may also be constructed in such a manner that it consists of a solid metal body which is highly heat conductive and onto which an electrically insulating but thermally highly conductive layer is applied. This layer may e.g. consist of silicon dioxide, diamond, or ceramic. The active electrodes can then be applied as layers as well onto the electrically insulating, thermally highly conductive layer.
The (half) ball-shaped electrode head provided by the invention as a whole has a smooth surface, and in the case of the formation of two or more electrode areas, these are immediately neighbouring with small insulating spacer strips, if required. Besides the electrode areas, the head comprises further areas which are formed by a highly heat conductive material in order to effectively dissipate the heat from the zone of the effective electrode areas into farther proximally situated zones of the system, where the heat is distributed over larger areas and volumes, thus preventing temperature peaks. According to preferred embodiments of the invention the electrodes can be applied as very thin layers onto a solid body. The layer thickness may be in the range of a few mm, e.g.  less than 5 mm.
According to a preferred embodiment of the invention the head consists of at least one electrode and, with the exception of a relatively thin electrically insulating layer which may be provided, exclusively of a highly heat conductive material.
Further preferred embodiment of the invention are described in the dependent claims.
In particular, it is provided to arrange several HF electrodes at the distal end of the head, with the individual electrodes being capable of optionally being controlled individually or in combination, i.e. as required, one or the other electrode can be switched off so that it does not emit HF energy. With such a configuration of the head it is possible to activate only those HF electrodes which are actually positioned with respect to the tissue to be treated in such a manner that the energy emitted by them has the desired effect, while those electrodes which would only cause an undesired heating of blood and tissue are not activated.
The invention also teaches certain dimensions of the head and the electrodes in accordance with preferred embodiments.